With a second thought, I think it would do perfectly with simply
averaging ALL channels, R, G, B, and A, for this cross-fade effect.

So I am reformulating my questions as follows: How do I produce the
blending mode when the foreground and background colors are averaged
channel-wise, for all channels, including the alpha channel?

Thanks

On Sun, Sep 27, 2009 at 4:25 PM, Oleg Krupnov <oleg.krup...@gmail.com> wrote:
> Paul, thanks for such an exhaustive answer!
>
> Here are answers to your questions:
> 1. My case is the simple one -- I'm rendering all text layers at once.
> 2. Yes and no. As I said, I am implementing the cross-fade effect,
> when one text fades out and another text fades in. The texts can be of
> the same color (simplest case) or of different color (different shades
> of gray), and their alphas change during the fading animation. The
> problem is that the semi-transparent areas of text, when overlapping,
> produce brighter pixels, the annoying "flash" effect, I'm trying to
> get rid of.
>
> My further research shows that kCGBlendModeLighten does not seem to
> apply to the alpha channel. Namely, colors are indeed composited using
> the lighten rule, but then the resulting color is alpha blended with
> the background as usual, so that two overlapping semitransparent
> pixels still produce a more opaque pixel, which is undesirable
>
> Am I missing the clue how to make kCGBlendModeLighten apply to the
> alpha channel as well?
>
> I have come to the following workaround: I fill the text layers with
> opaque black background. In this case the kCGBlendModeLighten works
> perfectly, as expected.
>
> Unfortunately, I am not fully satisfied with this solution, because I
> need the background of the text layers to be half-transparent as well.
> This returns me to the question of how to make kCGBlendModeLighten to
> the alpha channel.
>
> You said I could "average the color components but max the alphas."
> How can I do this? Thanks!
>
>
> On Sun, Sep 27, 2009 at 3:45 AM, Paul M <l...@no-tek.com> wrote:
>> This is a multi-pass operation - you wont find a single
>> blend mode that will do this in any simple manner.
>>
>> First question: Are you doing this all in one hit, or sequentially,
>> ie, rendering the first layer of text then adding subsequent
>> layers later?
>> Second question: is the text all the same colour and have the
>> same alpha component?
>>
>> If you're rendering all the text layers at once, you need to first
>> combine the text layers, then composite over the background. In the
>> simple case you describe, I imagine you'll need to average the colour
>> components but max the alphas. This should give you the results you
>> asked for.
>>
>> If you need to comosite different layers at different times, it
>> becomes a lot more complex.
>> If your text is not all the same colour then you need access to all
>> the previous layers so that they may be combined as above and the
>> composite redone.
>> If your text is all the same colour then you can write the alpha from
>> the first layer into the composite so that this may be extracted and
>> used later in the 2nd and subsequent composites. The process for
>> these later composites gets pretty gnarley, and you're really better
>> of keeping a reference to the preceeding layers if that's at all
>> possible.
>>
>>
>> paulm
>>
>>
>> On 27/09/2009, at 12:17 AM, Oleg Krupnov wrote:
>>
>>> Thanks!
>>>
>>> It seemed exactly what I need but I've found that it doesn't work as
>>> expected. The interpolated semi-transparent parts of the rendered text
>>> still appears brighter than it should.
>>>
>>> Namely, consider drawing white text on black background, and then
>>> drawing the same white text one more time over the first one.
>>>
>>> Suppose that when a letter is rasterized, one of the resulting
>>> interpolated pixels is semitransparent white, with alpha = 50%. After
>>> alpha blending with the black background, the pixel becomes a 50% gray
>>> pixel.
>>>
>>> Now when the second text is drawn on top, there will be also the same
>>> semitransparent white pixel with alpha = 50% at the same location.
>>> What result I'd like to achieve would be to have the same 50% gray
>>> pixel at this location. However, with kCGBlendModeLighten mode, what I
>>> think what's happening is that the system picks pure white as the
>>> lighter of two colors: 50% gray of the background and pure white
>>> (without taking into account the alpha of the new pixel), and then
>>> applies the usual alpha blending rule, which results in 75% gray.
>>>
>>> Is there a workaround? Or am I doing something wrong? Thanks!
>>>
>>> On Tue, Sep 22, 2009 at 7:23 PM, David Duncan <david.dun...@apple.com>
>>> wrote:
>>>>
>>>> On Sep 22, 2009, at 6:22 AM, Oleg Krupnov wrote:
>>>>
>>>>> Hi,
>>>>>
>>>>> I'd like to draw in a graphics context in such a compositing mode:
>>>>>
>>>>> R = MAX(S, D)
>>>>>
>>>>> i.e. out of two colors (source and destination), the maximum color
>>>>> (channel-wise) was chosen. This is basically equivalent to ORing the
>>>>> colors.
>>>>
>>>>
>>>> This is the Lighten blend mode (kCGBlendModeLighten). There doesn't
>>>> appear
>>>> to be a corresponding NS composting mode, and I'm not sure why, but you
>>>> can
>>>> easily get a CGContext from an NSGraphicsContext by asking it for its
>>>> graphicsPort, so this shouldn't be hard to integrate.
>>>> --
>>>> David Duncan
>>>> Apple DTS Animation and Printing
>>>>
>>>>
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>>
>
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